This invention concerns refractories and particularly insulating refractories.
Insulating refractories are known and are generally refractories with relatively high porosities (e.g., 50 volume percent or more). In other words, insulating refractory shapes are made by creating holes or voids within the shape to provide thermal insulation.
It is conventional practice today in producing most insulating refractory shapes to admix with the refractory material a substance, for example sawdust, which will burn out during firing and leave voids.
It is also known to form insulating refractories by incorporating air, for example in the form of a foam, into a slip or slurry of refractory particles, for example as set forth in U.S. Pat. Nos. 2,292,011, 3,232,772, and British Pat. No. 1,124,514. One of the main problems in this approach to producing insulating refractories is the stability of the foamed slurry. If it is exceedingly unstable, it may even collapse before the refractory shape is formed. In any case, it is essential that the foamed slurry maintain its structure and not collapse before it has dried and formed a semi-permanent, rigid structure. Also, the foamed slurry must not crack during the setting and drying steps.
(In this specification, the term "slip" refers to the mixture of water and solid ingredients, with or without a deflocculant, before foaming or aeration; the term "foam" refers to the air/water mixture, including a foaming agent, often referred to in the industry as a "preformed foam"; and the term "foamed slurry" refers to the mixture of "slip" and "foam"--sometimes referred to in the industry as "foamed slip"--and also, in discussing the background of the invention, to a "slip" which has been aerated in situ, for example by whipping in air.)
One solution to foamed slurry stability is to add an organic binder such as starch or polyvinyl alcohol to strengthen the foamed slurry. However, this has the disadvantage that foamed slurries containing such organic binders require a relatively long drying time, a matter of days, which is disadvantageous in mass production.
The present invention is directed toward the solution of the problem of producing a foamed insulating refractory shape which has a high strength-to-weight ratio and which is adapted to being made on a mass production scale.